Damping circuit for direct current measuring instruments



Mam}! 1956 E. E. LYNCH ETAL 2,737,627

DAMPING CIRCUIT FOR DIRECT CURRENT MEASURING INSTRUMENTS Original FiledJuly 13, 1948 Inventors: Edward ELyn c h Bernard D.i eete.

by A2604 4 w Their/Attorney.

United States Patent DAMPING CIRCUIT FOR DIRECT CURRENT MEASURINGINSTRUMENTS Edward E. Lynch, Wakefield, and Bernard D. Leete, Peabody,Mass., assignors to General Electric Company, a corporation of New YorkOriginal application July 13, 1948, Serial No. 38,488.

Divided and this application September 12, 1951, Serial No. 246,196

3 Claims. (Cl. 324-125) This application is a division of our copendingapplication Serial No. 38,488 entitled Frequency Type TelemeterReceiver, filed July 13, 1948, now Patent No. 2,629,008, issued February17, 1953, and assigned to the same assignee as the present invention.The invention relates to damping circuits for direct current measuringinstruments and has for its object the provision of an improved dampingcircuit which enables a much smaller damping capacitor to be utilized toobtain the same degree of damping previously obtainable only with a muchlarger and thus more expensive capacitor.

In our above-mentioned application Serial No. 38,488 there is discloseda frequency type telemeter receiver adapted to receive a transmittedalternating electric signal of relatively low frequency in theneighborhood of 6 to 27 cycles per second. This telemeter receiverfunctions to convert the alternating signal to unidirectional pulses ofconstant amplitude and duration occurring Whenever the alternatingsignal crosses the zero signal axis in either direction, and suppliesthese pulses to a direct current measuring means responsive to theaverage value thereof. The damping circuit for a measuring instrumentsuch as may be included in such direct current measuring means comprisesthe present invention.

The invention may be understood by reference to the followingdescription taken in connection with the accompanying drawing in whichthe single figure is a schematic circuit diagram of a telemeter receiverembodying the invention.

Referring to the drawing, at the right a transformer has a primary 1which may be connected to a source of alternating electric current suchas the usual 60 cycle electric outlet, and secondaries 2 and 3 connectedas shown to a full wave rectifier tube 4. A choke 5 and capacitor 6constitute a filter for reducing the A.-C. ripple in the rectifieroutput. This part of the circuit is a conventional D.-C. power supplyand may be replaced by any source of direct current voltage of propermagnitude to operate the vacuum tube stages hereinafter described.Resistor 7 and voltage regulator tube 8 further reduce A.-C. ripple andaccurately regulate the voltage applied to the vacuum tube anodes.

Capacitor 9 and transformer 10, at the left of Fig. 1 provide couplingmeans through which an input signal is received from the carrierequipment or other means by which the telemetric signal is conveyed fromthe trans mitter. A lower pass filter comprising an inductor or resistor11 and a capacitor 12 attenuates audio tones having a frequency abovethe range in which the telemeter receiver operates. These tones maycome, for example, from the carrier equipment, and preferably areeliminated by a low-pass filtering system, such as that shown, to avoidthe possibility that they may adversely aifect the operation of thereceiver.

Vacuum tube 13 and its associated circuit elements constitute a voltageamplifier stage. Resistor 14 and potentiometer 15 supply a fixed biaswhereby vacuum tube 13 is negatively biased almost to cut-off, andcapacitor 18 is a coupling capacitor for transmitting the A.-C.components of the output signal from vacuum tube 13 to the nextamplifier stage.

Vacuum tube 19 and its associated circuit elements constitute a secondvoltage amplifier stage. Resistors 20 and potentiometer 21 form avoltage divider supplying a negative voltage to bias this vacuum tubefar beyond cutoff. Capacitor 25 is a coupling capacitor for transferringthe A.-C. components of the output signal from vacuum tube 19 to thefollowing stages. Preferably, vacuum tubes 13 and 19 are both of thesharp cut-01f type.

The voltage amplifier stage associated with tube 13 functions to amplifythe positive half-cycle and a small portion of the negative half-cycleof a large inputvoltage signal; small signals being coupled to vacuumtube 19 without substantial distortion. Due to the cut-off biascondition of tube 19 and the shift in signal reference level occurringas the result of the elimination of D.-C. com ponents by couplingcapacitor 18, the amplifier stage associated with tube 19 functions toreject small amplitude signals completely and to limit the largeamplitude signals to provide a large amplitude signal-responsive outputvoltage having a substantially rectangular wave form of uniformamplitude. Reference may be had to the abovementioned application SerialNo. 38,488 for a more detailed description of the amplitude limitingaction of the amplifier stages associated with tubes 13 and 19.

Resistors 26 and 27 form a voltage divider whereby a part of the outputsignal from vacuum tube 19 is transmitted through resistor 28 to thegrid of vacuum tube 29, which together with load resistor 30 comprises avoltage inverter. Coupling capacitor 31 thus transmits through thefollowing stages a voltage which is degrees out of phase with thevoltage transmitted by capacitor 25.

Vacuum tubes 32 and 33 with their associated circuit elements constitutea push-pull amplifier stage. Signals from the preceding stages areapplied to the grids of these tubes through resistors 34 and 35respectively.

A pulse forming transformer has its primary 38 connected between theanodes of tubes 32 and 33. This transformer is of the saturable coretype having a magnetic core which saturates with relatively small valuesof current through the primary. Thus there is induced in secondary 39 ashort pulse of voltage each time the polarity of current through primary38 changes, according to the well known manner of operation of suchtransformers. These voltage pulses are applied thmrough resistors 41 and42 to the input of a rectifier 43, preferably of the bridge type asillustrated. The rectified voltage pulses then appear across resistor 44which is connected between the output terminals of the rectifier bridge.

Resistor 45 and D.-C. milliammeter 46 in series are connected inparallel with resistor 44. The reading of milliammeter 46 isproportional to the average value of the rectifier voltage pulses.Resistor 47 and variable resistor 48 supply a small adjustable buckingcurrent through meter 46 so that the meter can be made to give a zeroindication when the voltage pulses have an average value other than zerowhich represents Zero value of the quantity measured by the telemeteringsystem. Meter 46 may be an indicating instrument, a recorder, acontrol-initiating relay, an electrical system in which the outputcurrent is added to other currents for totalizing, or a combination ofsuch instruments. Resistor 4S helps to filter the rectified voltagepulses and thus aids in damping meter 46.

Resistors 48 and 41 are made adjustable as shown to provide means forcalibrating the zero and full scale readings respectively of meter 46.

Vacuum tube 49, resistors 45, 50 and 51, and capacitor 52 constitute adamping circuit for meter 46 to prevent undesirably large fiuctuationsof the meter needle Patented Mar. 6, 1956 mamas stoA-Q m nsatsptt amsasutedpulsest. is

damping should be sufficiently great to prevent any undesirable A.-C.fluctuation of the needle of meter 46. In the illustrated circuit, meter461is connectedjin the cathode -tocontrol grid circuit of tube 49,.andcapaeitor52 is connected between the grid-connnected side ofrneter-S andany connection'point, such as the anode of tube 49, in theanode-to-cathode circuit at, which an amplifier, phase-inverted replicaof the grid signal appears. This damping could be supplied by providinga very large capacitor in parallel with meter 46. However, with thecircuits shown, a much smaller capacitor 52 can be employed. Capacitor52 mustcharge to a higher voltage,

due to the voltage amplificationofvacuum tube 49, than would be the caseif the capacitor were connected directly in parallel with meter 46.Capacitor 52 thus provides an amount of damping substantially equivalentto that which would be provided by a much larger capacitor connected inparallel with meter 46 and having a capacitance value equal to thecapacitance of capacitor 52 multiplied by the voltage gain of vacuumtube 49.

Although we have described above one embodiment of our invention, manymodifications can be made, and we intend by the appended claims to coverall such modifications that fall within the true scope and spirit of ourinvention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In combination, means producing an output of electrical signalshaving alternating current components, a direct current instrument, andmeans for actuating said instrument with electrical signals which are ofsubstantially the average value of said output signals, comprising avoltage-amplifying vacuum tube having a cathode, anode and control grid,a capacitor coupling said control grid with said anode, a unidirectionalvoltage supply coupled serially with said cathode and anode, meanscoupling said instrument in series with said control grid and cathode,and means applying said output signals across said instrument.

2. In combination, means producing an output of peri odically-varyingsubstantially unidirectional signals, a direct current instrument, andmeans for actuating said device with electrical signals which are ofsubstantially the average value of said output signals, comprising avacuum tube having a cathode, anode and control grid, means couplingsaid instrument in series relationship with said cathode and controlgrid, means applying said output signals across said instrument, acathode-to-anode circuit for said tube including a source ofunidirectional voltage and an impedance coupled in series relationshipwith said cathode and anode for providing a cathode-anode voltage whichis an amplified and phase-inverted replica of said output signals, and acapacitor coupling said anode with said control grid.

3. In combination, means producing an output of periodically-varyingsubstantially unidirectional signals, a direct current sensing device,and means for actuating said device with electrical signals which are ofsubstantially the average value of said output signals, comprising avacuum tube having a cathode, anode and control grid, a load im-,

pedance and unidirectional voltage supply coupled in series relationshipwith said cathode and said anode, a capacitor coupling said anode withsaid control grid, means applying said output signals across saidcontrol grid and cathode, and means coupling said direct current deviceacross said control grid and cathode.

References Cited in the file of this patent UNITED STATES PATENTS2,179,263 Koch Nov. 7, 1939 2,271,478 Eldredge Jan. 27, 1942 2,412,782Palmer Dec. 17, 1946 2,629,014 Edwards Feb. 17, 1953 OTHER REFERENCESPublication I, Radio and Television News, vol. 48, issue 6, pp. 11 and20, Dec. 1952.

